The present invention relates to a base station, mobile station, communication system, transmission method and reordering method.
Currently, for mobile communication systems such as mobile telephones, third-generation type service using CDMA has begun, however, research of a next generation mobile communication system (LTE: Long Term Evolution) in which even faster communication is possible is being advanced by the 3GPP (3rd Generation Partnership Project) (refer to 3GPP, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTAN),” TR25.913 V7.3.0, Release 7, March 2006). Some of the large challenges in this research include increasing the transmission rate and reducing transmission delay.
In the LTE communication system, in order to increase the transmission rate and reduce transmission delay, the handover method used is designed to be at a higher level than that of a conventional system. In a mobile communication system, when a mobile station moves during communication, the base station that the mobile station is communicating with is switched (handover) according to the reception status. Therefore, in order to perform communication at increased transmission speed and with low transmission delay, improving the level of the handover is essential. In a LTE communication system, a packet exchange system is basic, so handover is a hard handover. In a hard handover, after the line connection with the base station that the mobile station communicates with before moving is cut, a line is connected between the mobile station and a target base station. In a hard handover, by obtaining system information about the target base station immediately before performing the handover, it is possible to perform the handover in a short time; however, transmission of user data becomes interrupted during the handover.
Therefore, in order to reduce transmission delay it is important that the state of interrupted transmission be shortened and that the loss of packets while transmission is interrupted be prevented. In the case that packets do become lost while transmission is interrupted, the lost packets are recovered in end-to-end retransmission of the packets, so transmission delay becomes large.
Therefore, in a handover in a LTE communication system, a method is specified in which among data that includes control information and packets for the mobile station, the transmission of packets that have not yet been transmitted to the mobile station from the source base station is taken over by forwarding those packets to the target base station from the source base station (refer to 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN),” TS36.300, Release 8, V8.0.0, April 2007).
Takeover During a Handover
FIG. 21 is a drawing explaining takeover during a handover. In (A) of FIG. 21, two base stations 1a and 1b are connected to a host station (for example a MME/SAE gateway) 2. A mobile station 4 exists within the cell 3a of the base station 1a, and is currently communicating with the base station 1a. In this state, as shown in (B) of FIG. 21, when the mobile station 4 moves in the direction toward the base station 1b and enters into the cell 3b, a handover is executed and the base station with which the mobile station 4 communicates is switched from the base station 1a to the base station 1b. Here, a base station that is in communication with a mobile station before the mobile station moves is called the source base station, and the base station that communicates with the mobile station after the mobile station moves is called the target base station.
The source base station 1a stores packets that were received from the host station 2 in an internal buffer, and sequentially sends the packets that are stored in that buffer to the mobile station 4. Therefore, when a handover occurs, the packets that are not sent to the mobile station are stored and exist in the buffer. In (B) of FIG. 21, it is necessary that before a handover, the packets n−2 to n be received and stored in the buffer without being sent to the mobile station, and that after the handover it is necessary that these packets be sent to the mobile station 4 from the target base station. Therefore, when executing the handover sequence, the source base station 1a transfers (forwards) the packets n−2 to n to the target base station 1b. By using this forwarding method, the target base station sends those packets to the mobile station 4 immediately after the handover, so no interruption in the packets occurs. Therefore, it becomes possible to execute high-speed handover without performing end-to-end retransmission of the packets. In the explanation above, n−2 to n are numbers (sequence numbers) that indicate the order of the packets.
Handover
FIG. 22 is a drawing explaining a handover in a LTE communication system, and FIG. 23 is a drawing explaining the handover procedure that is currently presumed for a LTE communication system.
Using a Measurement Report (report of the reception status of the base station 1 and other surrounding base stations), the mobile station 4 notifies the source base station 1 that a handover HO (Handover) is necessary (1. Measurement Control).
The source base station 1 decides a target base station 1b according to the contents of the Measurement Report (2. HO Decision), and sends a handover request (3. Handover Request) to that target base station 1b. At that time, the source base station 1a also sends information about the mobile station (mobile station ID, QoS (Quality of Service) information, etc.). The target base station 1b performs call-receiving control based on that information (4. Call-Receiving Control).
After allowing the acceptance of the mobile station, the target base station 1b returns a handover response to the source base station (5. HO Response). After that, the source base station 1a sends a handover instruction to the mobile station 4 (6. HO Instruction), then immediately afterwards starts taking over the data (packets) (Packet transfer: Forwarding).
The mobile station 4 receives the handover instruction, and then obtains synchronization using L1/L2 signaling (7. Synchronization), and after synchronization has been obtained, sends a handover complete report to the target base station 1b (8. HO Complete).
After this, the target base station 1b sends a handover complete report to the host station 2 (9. HO Complete). After receiving the handover complete report, the host station 2 changes the packet transmission path from the source base station 1a to the target base station 1b (10. Path Change), and returns a HO complete response to the target base station 1b (11. HO Complete Response). The target base station 1b uses a HO complete response to notify the source base station 1a that the handover HO is complete (12. Resource Release). After that, the path between the source base station 1a and the host station 2 is eliminated (13. Resource Release).
Packet Order Alignment Control
When packets are forwarded (forwarded) during execution of the handover sequence described above, there is a possibility that packets that are transferred by the target base station 1b will be jumped over by the packets that flow from the host station 2 resulting in the sequence numbers becoming mixed up. When packets are transferred from the target base station 1b to the mobile station 4 as are with the sequence numbers mixed up, the mobile station 4 is unable to receive the packets in the correct order, so the quality of communication is degraded, and as a result high-quality communication cannot be achieved just before and after a handover.
Therefore, in the LTE communication system, by using a method as described below, the base station and mobile station are capable of maintaining packet order. FIG. 24 is a drawing explaining packet order alignment wherein the target base station 1b maintains the packet order by sending packets that are transferred from the source base station 1a with higher priority than packets that are received from the host station.
In FIG. 24, before a handover, packets n−5 to n are stored in the source base station 1a, and of these packets, packets n−5 to n−2 are sent to the mobile station 4, however, packets n−1 and n are not sent to the mobile station 4. In addition, of the packets that are sent to the mobile station 4, packets n−5 and n−3 are not rightly received by the mobile station 4 (NACK), and packets n−4 and n−2 are rightly received (ACK). Therefore, the mobile station 4 save the packets n−4 and n−2 in a buffer BF1, and does not save the packets n−5 and n−3.
When a handover occurs in this state, the source base station 1a transfers (forwards) the packets n−5 and n−3, which were not rightly received by the mobile station 4, and the packets n−1 to n, which have not yet been sent to the mobile station 4, to the target base station 1b, and the target base station 1b stores those packets in a buffer BF. Also, after the handover, the host station 2 sends two packets m to m+1 that are intended for the mobile station 4 to the target base station 1b, and the target base station 1b stores those packets in the buffer BF.
After communication with the mobile station 4 becomes possible, the target base station 1b first sends the packets n−5, n−3 and n−1 to n that were forwarded from the source base station 1b to the mobile station 4. Next, the target base station 1b sends the packets m to m+1 that were received from the host station 2. As shown in FIG. 25, the mobile station 4 rearranges the sequence order of the packets n−4 and n−2 that were received before the handover, and the packets n−5, n−3, n−1 and m to M+1 that were received after the handover, and gives those packets in order to an upper layer.
In the explanation above, the case was explained in which all of the packets n−5, n−3 and n−1 to n were transferred (forwarded) to the target base station 1b, however, in some cases, only the packets n−5 and n−3 will be transferred, and transferring of packets n−1 to n will be delayed. FIG. 26 is a drawing explaining packet order arrangement in that case. The target base station 1b stores the transferred packets n−5 and n−3 together with the packets m to m+1 that come from the host station 2 in a buffer BF, however, sends the packets n−5 and n−3 that were forwarded from the source base station 1a to the mobile station 4 first. After that, in the case that packets n−1 and n are forwarded from the source base station 1a later, the target base station 1b monitors whether a set time (waiting time) has elapsed, and when the packets n−1 and n still have not been forwarded from the source base station 1a even after the waiting time has elapsed, the target base station 1b determines that forwarding has been completed and sends the packets m and m+1 that were received from the host station to the mobile station to the mobile station. Even though the packets n−1 and n may be received from the source base station 1a after forwarding has been completed, the target base station 1b discards those packets.
The mobile station 4 executes a process for rearranging the order of the sequence numbers of the received packets (reordering). As shown in FIG. 27, the mobile station 4 rearranges the sequence numbers of the packets n−4 and n−2 that were received before the handover, and the packets n−5, n−3, m and m+1 that were received after the handover, and gives those packets in order to an upper layer.
Protocol Configuration
As described above, in a handover in a LTE communication system, packet transfer (forwarding) and packet reordering are necessary techniques. The relationship between these functions will be explained in more detail here.
FIG. 28 is a drawing explaining the protocol configuration between a mobile station and a base station. Between a mobile station and a base station there is at least a PDCP (Packet Data Convergence Layer) layer, RLC (Radio Link Control) layer and a lower layer (MAC layer/physical layer MAC/PHY). A packet routing function or the like is provided in MME/S-GW.
The main features of each protocol are as described below.
(1) PDCP: In the PDCP layer, the transmitting side compresses the upper protocol header, as well as attaches a sequence number and performs transmission. The receiving side checks the sequence number, and by doing so, performs a discarding process for redundant reception. Retransmission is not performed in the PDCP layer.
(2) RLC: The RLC layer is a layer having a retransmission function, and in the RLC layer, a sequence number is newly attached to the data that is different from the sequence number that is attached to the data from the PDCP layer, and the data is then transmitted. For example, when data having a sequence number n is received from the PDCP layer, that data is divided into a plurality of data, and to each division of data, sequence numbers I(1), I(2), I(3), . . . are attached in the RLC layer, after which the data is transmitted. The receiving side notifies the sending side by using those sequence numbers I(•) to send a transmission confirmation (Ack/Nack signal) indicating whether the data were received rightly or improperly. When an Ack signal is returned, the sending side deletes the data that is saved, however, when a Nack signal is returned, the sending side retransmits the saved data.
(3) Lower Layer                MAC: The MAC layer is a layer that multiplexes/demultiplexes the data of the RLC layer. In other words, the sending side multiplexes the data of the RLC layer and transmits the data, and the receiving side demultiplexes the received data of the MAC layer to RLC layer data.        PHY: The PHY layer is a layer for transmitting and receiving data by radio signals between the user terminal 4 and base station 1, and converts MAC layer data to radio data, or converts radio data to MAC layer data.        
Data destined for a mobile station first flows from the upper layer (for example IP layer) to the PDCP layer to become a PDCP SDU (Service Data Unit), then header information (PDCP layer sequence number, etc.) is attached to become a PDCP PDU (Protocol Data Unit).
The PDCP PDU is sent to the RLC to become a RLC SDU, where header information (RLC layer sequence number, etc.) is further attached to become a RLC PDU. The RLC PDU passes through the lower layer processing after which it arrives at the RLC layer of the mobile station. In this RLC layer, the header is deleted and the RLC SDU is reassembled, then in the PDCP layer, the PDCP PDU header is deleted to become a PDCP SDU, then the data is sent to the upper layer.
In this kind of protocol configuration, in a LTE communication system, the forwarding of packets is performed in PDCP SDU units, and reordering is performed in PDCP PDU units. When forwarding is performed in PDCP SDU units, header information such as a sequence number is not attached to the PDCP SDU unit packet, so that sequence number is not forwarded.
Therefore, in a case where the forwarding is performed in PDCP SDU units, it is necessary to forward PDCP SDU data and harder information including the sequence number separately.
The RLC SDU data and PDCP PDU data are essentially the same data, so in the specification for the present invention, unless specifically specified, these will simply be called packets, and when a packet number is provided, that number is the sequence number of the PDCP PDU data.
Operation of the Source Base Station
FIG. 29 is a flowchart of the operation of a source base station during a handover.
When the source base station 1a receives the reception field strength from the user terminal 4 by way of a Measurement Report (step 101), the source base station 1a determines whether or not a handover HO is necessary (step 102), and when a handover is not necessary, returns to the beginning.
However, when it is determined that a handover is necessary, the source base station 1a decides a target base station 1b according to the contents of the Measurement Report, and sends a handover request to that target base station 1b (step 103).
After that, the source base station 1a receives a handover response that is sent from the target base station 1b (step 104), and forwards the remaining packets to a target base station (step 107).
After receiving a resource release message that is sent from the target base station 1b (step 108), the source base station 1a executes the resource release (step 109).
Operation of the Target Base Station
FIG. 30 is a flowchart of the operation of a target base station during a handover.
After receiving a HO request from the source base station 1a (including the mobile station ID, QoS information, etc.) (step 121), the target base station 1b performs call receiving control based on that information, and determines whether to allow the acceptance of the mobile station (step 122), and when the mobile station is not allowed, performs post processing (step 130) and ends handover control.
On the other hand, when acceptance of the mobile station is allowed, the target base station 1b returns a HO response to the source base station 1a (step 123). The target base station 1b then stores the packets that are forwarded from the source base station 1a in a buffer (step 124), and receives a HO complete report from the mobile station 4 (step 125). After receiving the HO complete report, the target base station 1b sends a HO complete report to the host station 2 (step 126). The host station 2 receives the handover complete report, then changes the packet transmission path from the source base station 1a to the target base station 1b, and returns a HO complete response to the target base station 1b. After receiving the HO complete response from the host station 2 (step 127), the target base station 1b starts sending the packets that were forwarded from the source base station 1a to the mobile station preferentially, and after those packets have been sent, sends the packets that were received from the host station 2 to the mobile station (scheduling: step 128). In addition, at the same time as step 128, the target base station 1b sends a resource release to the source base station 1a (step 129), then performs post processing (step 130) and ends handover control. In the scheduling process of step 128, when forwarding of packets from the source base station 1a is delayed, the target base station 1b monitors whether a set time (Waiting Time) has elapsed, and when no packets have been transferred even though the Waiting Time has elapsed, the target base station 1b determines that forwarding has finished and sends all the packets that were received from the host station 2, and even though packets may be received from the source base station 1a after forwarding has finished, the target base station 1b discards those packets.
Operation of a Mobile Station
FIG. 31 is a flowchart of the operation of a mobile station during a handover.
The measurement unit of the mobile station 4 sends a notification of the reception field strength or the like to the source base station by way of a Measurement Report (step 151). After that, the mobile station 4 waits for a HO instruction from the source base station 1a, and after receiving a HO instruction (step 152), obtains synchronization with the target base station 1b using L1/L2 signaling (step 153), and after synchronization has been obtained, sends a handover complete report to the target base station 1b (step 154), then, in the case where packets are received from the target base station 1b, the mobile station 4 executes a reordering process (steps 155 to 160).
In other words, when the control unit of the mobile station receives lower layer packets from the target base station 1b, the control unit creates RLC SDU data and delivers that RLC SDU (PDCP PDU) data to a reordering unit (step 155). The reordering unit checks whether there are any missing sequence numbers (step 156), and when there are no missing sequence numbers and the sequence numbers are continuous, delivers PDCP SDU data obtained by removing a header from the RLC SDU (PDCP PDU) data to the upper layer (step 160). However, when there is a missing sequence number, the control unit instructs the reordering unit to save the RLC SDU (PDCP PDU) data. By doing so, the reordering unit saves the PDCP PDU data (step 157), and checks whether RLC SDU (PDCP PDU) data having a continuous sequence number has been received (step 158). When RLC SDU (PDCP PDU) data having a continuous sequence number has been received, the reordering unit delivers PDCP SDU data obtained by removing a header from the RLC SDU (PDCP PDU) data to the upper layer, a well as delivers PDCP PDU data obtained by removing a header from the saved PDCP PDU data to the upper layer (step 160).
However, in step 158, when RLC SDU (PDCP PDU) data having a continuous sequence number is not received, the reordering unit monitors whether a preset amount of time has elapsed (step 159), and when that set amount of time has not elapsed, repeats the process from step 157; however, when the set time has elapsed, the reordering unit delivers PDCP SDU data obtained by removing a header from the saved PDCP PDU data to the upper layer even though the sequence numbers may not be continuous (step 160).
Problems
In a LTE communication system, the following problems exist when executing packet forwarding during a handover. That is, as was described above, when a handover is executed in a LTE communication system, a process for taking over the packet for the mobile station that remains at the source base station 1a is executed, and packets are forwarded to a target base station during that taking over process. However, in the handover control described above, if the value of the waiting time by the target base station 1b is small, the target base station 1b begins sending the packets received from the host station regardless of whether or not all of the packets have been forwarded, so a problem occurs in that the packets that have not yet been forwarded are discarded. On the other hand, if the value of the waiting time is large, there is a problem in that the target base station 1b cannot send the packets received from the host station until the waiting time has elapsed even though all of the packets may have already been forwarded, so a transmission delay occurs. In other words, in conventional handover control there is an increase in communication delays and throughput is degraded, making it impossible to maintain high-quality communication immediately before and after a handover.
As first related art is a method of notifying the target base station of the last packet to be forwarded from the source base station (refer to Samsung, “Method to Release Resources at Source ENB During Handover,” R3-061032, RAN3#53, September 2006). When forwarding is delayed, the target base station that has been notified of the last packet can transmit the packets received from the host station by starting the sequence numbers of those packets from the sequence number of the last packet+1. In addition, by comparing the sequence numbers of the packets that are forwarded with the sequence number of the last packet, it is possible to forcibly end the waiting time and to detect the end of forwarding with optimum timing. However, in the case that the last packet is deleted during forwarding, the target base station will be unable to accurately detect the last packet.
Moreover, as second related art is a mobile communication system for making high-speed packet data transfer possible with no data loss during a handover between base stations during high-speed packet communication (refer to Japanese patent publication No. JP2004-282652A). When a handover occurs in this mobile communication system, the source base station of the handover transfers (forwards) packet data to the target base station of the handover. However, there is an increase in communication delay due to reordering by the mobile station, and there is no improvement in the degraded throughput.
Furthermore, as third related art is a method in which the target base station jumps the packets sent from the host station without waiting for forwarded packets to arrive (refer to Japanese patent application No. 2006-086537A). In this method, by distinguishing the packets that are received from the source base station from the packets that are received from the host station, transmission is possible in which the packets jump. However, it is necessary for the mobile station to have two order control functions, and the control thereof becomes complicated.